Angel Pagan rounds third base to score on his inside-the-park homer to beat the Rockies in the 10th inning May 25. The home run occurred after Pagan injured his hamstring in that game.

Photo: Jeff Chiu, Associated Press

Angel Pagan rounds third base to score on his inside-the-park homer...

Image 2 of 3

Pagan will miss most of the remainder of the regular season and could be out for the year if complications arise.

Photo: Brant Ward, The Chronicle

Pagan will miss most of the remainder of the regular season and...

Image 3 of 3

SAN FRANCISCO, CA - MAY 21: Angel Pagan #16 of the San Francisco Giants returns to the dugout during the fourth inning against the Washington Nationals at AT&T Park on May 21, 2013 in San Francisco, California. The San Francisco Giants defeated the Washington Nationals 4-2 in 10 innings. (Photo by Jason O. Watson/Getty Images)

LOS ANGELES - News that center fielder Angel Pagan will have surgery seemed fait accompli. The prognosis is what provided the jolt.

Giants manager Bruce Bochy said Monday that after Pagan undergoes an operation Tuesday to repair the hamstring tendon behind his left knee, he will be lost for about 12 weeks, not the six to eight originally expected.

That means Pagan will miss almost the entire remainder of the regular season and could be lost for the year if complications arise. Twelve weeks from Tuesday is Sept. 17, when the Giants have 12 games remaining.

On the other hand, with a quicker-than-expected recovery, Pagan could be back in 10 weeks after an injury he sustained a month ago is repaired.

Either way, the Giants will have to charge into the stretch drive without a key player from their 2012 championship team, who signed a four-year, $40 million contract after the World Series win.

"It's a loss. There's no getting around it," Bochy said. "When you lose your leadoff hitter, the guy who led off for you all the way through the World Series and played a significant role helping you win a World Series, there's no question it's a loss."

Pagan was expected back in the lineup over the weekend until he reinjured the tendon during a minor-league rehab start. Instead, he will have surgery at the Kerlan-Jobe clinic, performed by Dodgers orthopedist Dr. Neal El Attrache. It is not unusual to have another team's surgeon perform an operation.

Bochy said the Giants plan to have Gregor Blanco be the primary center fielder, with Juan Perez in the mix. Bochy has had no conversations with general manager Brian Sabean about promoting Roger Kieschnick, a corner outfielder who has hit well all season at Triple-A Fresno, or Gary Brown, the 2010 first-round pick who struggled for the first two months before going on a hot streak.

Bochy made a statement about Blanco's future as the regular leadoff hitter by batting him there Monday night against a left-hander, Hyun-Jin Ryu. Blanco took a .304 average into the series opener at Dodger Stadium.

Blanco is a better defensive center fielder than Pagan. After Saturday's game, Blanco spoke passionately about his hitting, too.

"I know what I'm capable to do," he said. "I know I can do good things offensively. A lot of people talk about my defense, my speed, the way I run the bases. I want people to talk about my offense. I know I can do some damage."